Cytokinesis is a stage in cell division. Cell division is a very common activity in your body -- occurring billions of times an hour. The vast majority of those cell divisions are in a process called mitosis, which is the same for males and females. Both males and females also use cell division to produce gametes -- sperm or eggs, respectively -- in a process called meiosis. Meiosis in the female results in one large egg and three small entities called polar bodies, with an unequal division sending most of the cytoplasm to the egg.

Cell Division

The final stage in cell division is cytokinesis. That step occurs after the DNA has been duplicated and arranged within the cell. In mitosis and in meiosis in the male, cytokinesis is a symmetric process. A cleavage furrow appears around the middle of the cell, and it gradually pinches tighter, splitting the cell in half. The cytoplasm -- the jelly-like material inside the cell -- divides equally between the two daughter cells.

Meiosis I

The first stage in cell division is the duplication of all a cell's DNA. In meiosis, the cell division that results in egg or sperm cells, the initial cell starts with 23 pairs of chromosomes -- that is, 46 separate strands of DNA. After duplication, the cell has 46 pairs of chromosomes, and sends 23 pairs of chromosomes to each side. Then the cell divides. In meiosis in the female the final step, the cytokinesis, is shifted way over to one side. So each of the resulting daughters has a full set of 23 pairs of chromosomes, but one has hardly any cytoplasm, while the other has almost all. The one with hardly any cytoplasm is called a polar body.

Meiosis II

The next cell division of meiosis happens like the first, except for one key step: the DNA does not duplicate. Instead, the 23 pairs of chromosomes separate from each other. Each side of the cell then has 23 single chromosomes -- that's half the number in a normal cell, so the two daughters are called haploid. Once the chromosomes have separated, the cell divides again. And once again, in the female, cytokinesis is very asymmetric. One part gets almost all the cytoplasm while the other gets almost none. Again, the small portion is called a polar body. The initial cell divided into one polar body and a larger daughter cell. Each of those two divide again. The polar body divides into two daughter polar bodies, and the larger daughter cell divides into another polar body and an egg cell.

Why the Difference?

The unequal division of cytoplasm results in several advantages. One is that the polar bodies, without enough cytoplasm to feed themselves, degrade away, meaning the mother is not burdened with more embryos than she can reasonably carry. Another is that a fertilized egg will need to support itself for some time. It takes a while for the fertilized egg to attach itself to the mother's uterus and obtain nourishment for the developing embryo. That means the fertilized egg needs to contain enough resources to sustain cell division for at least a few days.

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About the Author

First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.